When image signals are read from the image sensor of an ordinary video camera, electric signals generated in several tens of thousands to several millions of pixels are read by scanning every one row of pixels and sequentially reading them from one electric signal read-out line. Therefore, it takes considerable time in reading the image signal, which makes it difficult to gain the recording speed.
On the other hand, an ordinary high-speed video camera achieves high-speed recording by providing a plurality of read-out lines on an image sensor, and scanning and reading out a plural number of rows of pixels at once.
To further speed up the recording out, all the pixels are processed in parallel. That is, several to several tens of electric signal-storage elements are arranged in the vicinity of each of all the pixels, and electric signals generated at once in the photodiodes for all the pixels are sequentially transferred to the electric signal-storage elements, and thus, the recording speed can be further increased.
For example, U.S. Pat. No. 5,355,165 discloses an image sensor which comprises, as shown in FIG. 19, one longitudinal photodiode 3, a CCD type memory and a horizontal read-out CCD 9, which are arranged for every one pixel 1. The CCD type memory comprises 6 rows of horizontal transfer CCDs 5 and 5 rows of vertical parallel CCDs 7 for memory. This image sensor achieves continuous record of 30 image frames achieved at a recording rate of several hundreds of thousands of sheets/sec.
However, the image sensor shown in FIG. 19 has a problem in that a plurality of electric signal-storage elements are needed for each of the pixels 1 in order to increase the number of picture sheets of continuous recording, which induces a further problem in that the pixel must have a larger size so as to increase the number of electric signal-storage elements. However, limitation in the area of a chip for an image sensor exists, and therefore, the number of pixels becomes smaller as the size of the pixel becomes larger, so that the spatial resolution of the image sensor degrades. To solve this problem, it is considered to provide a plurality of electric signal-storage elements extending downwardly from and perpendicular to an image sensing plane composed of pixels. However, in view of the level of the present technology, it is impossible to construct an image sensor having such a three-dimensional structure, using a single or one chip.
By contrast, there is proposed an image sensor which is constructed by laminating a single planar chip having an array of photodiodes formed thereon and a plurality of planar chips each having a plurality of electric signal-storage elements formed thereon to each other, and which transfers charges generated in the photodiodes in the vertical direction (i.e., the thickness direction of the chips).
Further, Japanese Laid-Open Patent Publication No. 3-50973 discloses an image sensor as shown in FIG. 20. The image sensor comprises a plurality of chips 11, each of which has a row of photodiodes 13 arranged on one end face thereof, and digital memory elements 15 provided to the photodiodes 13 at one to one. These chips 11 are laminated on one another to construct the image sensor. Every chip 11 of this image sensor functions as a linear sensor capable of continuously storing brightness information corresponding to several thousands of points in time. Therefore, the image sensor constructed by laminating a plurality of such chips 11 that serve as linear sensors can record several hundreds of consecutive picture sheets at ultra high speed. Light 17 comes into the photodiodes 13 on the end faces of the laminated chips 11 from a direction orthogonal to the faces of the chips 11.
However, it is difficult in view of the level of the present technology that the above-mentioned image sensor, constructed by laminating one planar chip having an array of the photodiodes formed thereon and the plurality of planar chips each having the electric signal-storage elements formed thereon to each other, carries electric charges in the vertical direction between each of the chips, with low noises.
With respect to the image sensor shown in FIG. 20, it is difficult to construct a photodiode with high accuracy and low noises capable of responding to the light 17 coming into the end face of the chip 11 from the direction parallel to the chip surface. Further, the image sensor of FIG. 20 requires a very large number of chips to arrange a photo-receptive area having a sufficient size or a sufficient number of pixels.
An object of the present invention is therefore to provide an image sensor capable of recording a sufficient number of continuous image frames and achieving high-speed recording with high accuracy and low noises.